This invention generally relates to reticle inspection and more particularly relates to reticle stage design for actinic inspection of EUV reticles.
Reticles, i.e., photoreticles or photomasks, are transparent masks that allow electromagnetic radiation to pass in a defined pattern in a step in photolithography for preparation of integrated electronic circuits.
Over the years, component density has dramatically increased in integrated circuits until components can be measured in nanometers. The most recent commercial applications in photolithography employ very short wavelengths of radiation, e.g., EUV or extreme ultraviolet having wavelengths. “EUV” is an abbreviation for extreme ultraviolet light having wavelengths of less than 20 nm down to soft x-rays.
The extremely small size of components has led to extreme reticle requirements. Among problems encountered are issues with contamination. At such small sizes, contaminating particles in nanometer sizes can disrupt the integrity of the reticle and thus ruin resulting integrated circuits.
As a part of the preparation of such reticles, inspection is thus required. Unfortunately the inspection process itself can introduce contamination thus ruining or at least require cleaning to the reticle.
Thus one of the main requirements of a EUV photo-mask inspection is that no particles/contaminants are added to the photo-mask (reticle) during the inspection. Along with the exacting precision requirements demanded for inspecting EUV reticles, this places rather special requirements, especially on the reticle stage which forms the single largest moving component inside the chamber used for the inspection of the reticle.
Existing inspection apparatus has not met the demands of reticles formed using EUV since there exists too much probability that the inspection process will contaminate the reticle.
As an example, a current inspection apparatus includes an X stage and carries the reticle. The X stage rides on two Y stages that provide motion in both the Y and Z directions. The stages use pre-loaded air-bearings against mapped precision surfaces to constrain the other degrees of freedom and use interpolating encoders for feedback. Fine adjustments in the Y direction and auto-focus in the Z direction are provided by actuation/sensing built into the optical train.
The existing approach to positioning the reticle over the optics is limited in both precision and architecture and cannot be extended to the actinic inspection of EUVL reticles for the following reasons:                a. The design was created for use at atmospheric pressure and for a significantly less demanding contamination requirement that does not scale up to the requirements for EUV actinic inspection.        b. It uses air-bearings that would prove both a technical and economic challenge in being integrated into the vacuum chamber, and        c. The architecture does not have either the resolution or precision to address the requirements posed by EUV mask inspection.        